DNA methylation is essential for normal mammalian development and plays critical roles in various biological processes, including genomic imprinting, X-chromosome inactivation and repression of transposable elements. Although DNA methylation patterns are relatively stable in somatic cells, global reprogramming of DNA methylation occurs during mammalian preimplantation development. Advances in DNA methylation profiling techniques have been revealing the DNA methylation dynamics in mammalian embryos. Recently, we and other groups reported genome-scale DNA methylation analyses of human oocytes and preimplantation embryos, highlighting both the similarities and differences in the DNA methylation dynamics between humans and mice. In this review, we introduce the current knowledge of DNA methylation dynamics during early mammalian development. We also discuss the possibility of the application of genome-scale DNA methylation analysis techniques to human gametes and embryos for diagnostic purposes.

A new prenatal genetic testing technique using cell-free fetal DNA in maternal plasma, the Non-Invasive Prenatal Test (or NIPT), was introduced in Japan in 2013. The NIPT is easy and safe but not definitive; it also has high sensitivity and specificity under certain conditions. In the near future, it could be used not only for detecting chromosomal aneuploidy, but also complete genome analysis of the fetus. However, termination of pregnancy is the only option for those wanting to avoid having a baby with a chromosomal anomaly or a genetic disease. Given this situation, this review explores the ethical and social issues surrounding prenatal genetic testing focusing on three points: 1) selective abortion as an ethical issue, 2) informed consent and decision making, and 3) alternative perspectives on prenatal testing.

Purpose: To highlight the imperative of informed consent in the fertility preservation of cancer patients before ovarian tissue autotransplantation. Methods: Three papers of tumor recurrence after autotransplantation of frozen-thawed ovarian tissue were compared with the main papers before tumor recurrence was reported in the cancer patients. Results: Histology was performed before autotransplantation in cases 1 and 3, but not in case 2. Histology alone is insufficient for the detection of minimal residual disease (MRD). Furthermore, transplanted ovarian tissue is different from that examined for MRD detection, and how much of the resected ovarian tissue was examined for MRD detection is unclear. The possibility that grafted tissue caused tumor recurrence cannot be ruled out in any of the three cases, because autotransplantation in cancer patients, at present, uses ovarian tissue different from that examined for MRD. Conclusions: We recommend informed consent for cancer patients, because: 1) the transplanted ovarian tissue is different from the ovarian tissue examined for MRD detection; 2) the amount of resected ovarian tissue analyzed for MRD is very small; and 3) MRD detection methods vary. In conclusion, freezing and storage of ovaries should be encouraged, transplantation must be performed carefully, and informed consent is essential.

Mammalian primordial germ cells (PGCs) are specified in the early post-implantation embryo. Attempts have been made to establish in vitro PGC development since the derivation of embryonic stem cells (ESCs) from blastocysts. Despite the advances made with mouse models, similar studies in human germ cell development have not progressed because practical and ethical reasons prevent the use of early human embryos. Recently, we and others developed a robust in vitro system for producing human primordial germ cell-like cells (hPGCLCs) from ESCs and induced pluripotent stem cells (iPSCs) by inducing competency for germ cells. Strikingly, the molecular mechanism for germline differentiation is not fully conserved between mouse and human, probably because of the differences in their early embryogenesis and regulation of the pluripotent state. Here, we present a review of the current status in the field of in vitro germ cell production from pluripotent stem cells, and discuss how its usefulness could be extended to clinical applications.

Mitochondrial DNA (mtDNA) mutation is associated with serious human disorders and affects multiple organs and tissues with high-energy requirements. Since the transmission of mtDNA is complex and is not fully understood, an accurate estimation of mtDNA disease transmission by preimplantation genetic diagnosis (PGD) or by prenatal diagnosis (PND) remains challenging. Recently, nuclear transfer techniques, including maternal spindle transfer (MST), pronuclear transfer (PNT) and polar body transfer (PBT), have shown the promising results. These methods avoid the transmission of mutated mtDNA from mother to offspring, and are collectively known as the mitochondrial replacement therapy (MRT). Further, the United Kingdom Parliament approved the Human Fertilisation and Embryology Authority (HFEA) to grant licenses for experimental use of MST and PNT in humans in 2015. Thus, a new era of assisted reproductive technology (ART), in which cures can be provided at the gamete or early zygote stages, is realistically approaching. In this review, we summarize the methods and the challenges confronting the clinical application of MRT.

Rat 1-cell embryos fertilized in vivo or in vitro show a complete developmental block at the 2- to 4-cell stage in conventional culture media. To develop a culture medium supporting early embryonic development in rats, we examined the effects of some chemical and physical factors on the development of rat 1-cell embryos. The results indicated that (1) the developmental block is caused by phosphate, (2) glucose and amino acids are necessary, (3) the optimal osmolarity is 244–246 mOsm, and (4) the presence of bovine serum albumin and relatively high (110.0–130.0 mM) NaCl concentrations in the early stages preceding pronucleus formation is necessary. On the basis of these results, novel culture media designated as modified rat 1-cell embryo culture medium (mR1ECM) and mR1ECM for in vitro fertilization (mR1ECM-IVF) were developed. In cultures using these media, 90% or more of rat 1-cell embryos fertilized in vivo or in vitro successfully developed to the blastocyst stage in vitro.

Systematic studies of mouse embryo culture beginning in 1949 led to an understanding of essential medium components for early mammalian embryos, and embryo culture from the zygote to the blastocyst stage was achieved in 1968. Since then, medium components that are either beneficial or detrimental for embryo culture have been identified. A variety of culture media that mimic the female reproductive tract, such as human tubal fluid medium and sequential media, were developed from the 1970s to the 1990s, and a single medium in which the concentrations of components were determined by a simplex optimization method was introduced for clinical use in 2002. While either sequential media or a single medium is currently used in most cases, no conclusion has yet been reached as to which of the two approaches is the best. That we are now easily able to culture embryos is the result of the work of pioneers. This review presents a chronological overview of media development from initial attempts at mouse embryo culture using synthetic media to the human embryo culture media used today. It also presents the characteristics of sequential media and a single medium. Finally, problems observed with current embryo culture media are discussed, along with future development in this area.

The TYH medium was first reported as a medium for in vitro fertilization (IVF) of mouse eggs with epididymal spermatozoa, by Toyoda, Yokoyama and Hosi, in 1971. It was a modified Krebs-Ringer bicarbonate solution, supplemented with glucose, Na-pyruvate, antibiotics and bovine serum albumin. In TYH medium, almost all eggs are fertilized within 1 h, if the spermatozoa are pre-incubated for 2 h before insemination, while the sperm penetration is delayed for approximately 1 h when fresh epididymal spermatozoa are used. These findings showed that sperm capacitation can be induced in a chemically defined medium without requiring the female reproductive tract. Although the medium was not specifically named in the original paper, it was later called “TYH” after the initials of the three authors of the original paper. The IVF method using TYH medium is widely used due to its high reproducibility. In this mini-review, we describe the early efforts to develop the TYH medium and briefly discuss the related areas.